The present invention relates generally to a gas fired turbomachine coupled to a heat recovery steam generator (HRSG); and more particularly to a modified bottoming cycle for fuel gas saturation and heating to increase power output and thermodynamic efficiency.
Some power plants utilize a gas turbine that is thermally coupled to a HRSG. The HRSG may be a non-contact heat exchanger that allows feed water for a steam generation process to be heated by exhaust gases of the gas turbine. The HRSG has a duct with tube bundles interposed therein such that the feed water is evaporated to steam as the gas turbine exhaust gas passes through the duct. The efficiency of this arrangement lies in the utilization of the otherwise wasted exhaust gas energy.
Modern HRSGs typically employ multiple pressure sections to recover the maximum energy from the exhaust gas. For example, the HRSGs utilized to recover the exhaust gas energy from advanced heavy-duty industrial gas turbines (commonly referred to as the F, G or H-Class machines) have three sections where steam is generated at high, medium (or intermediate), and low pressures. Earlier variants typically have two pressure levels. Furthermore, the aforementioned HRSGs also have a reheater section, where steam returning from the high pressure (HP) steam turbine is reheated to the same temperature as the HP steam for better efficiency. As such, the installation costs associated with multiple pressure reheat HRSGs are considerably higher than earlier HRSGs with two or even single pressure level and no reheat (commonly used with earlier generation heavy-duty industrial GTs also known as “E-Class” machines). The structural configuration and thermal flow paths are quite complex. The operation of modern HRSGs with multiple pressure levels is considerably more involved than a simple waste heat recovery boiler, including evaporator (boiler) drum level and pressure controls, temperature control for steam using attemperation via desuperheating water sprays and low pressure (LP) economizer recirculation for maintaining the economizer tube metal temperature above a minimum to prevent corrosion.
For the foregoing reasons, there is a desire for a system that provides the highest possible efficiency and output with the simplest and cost-effective HRSG configuration. The system will ideally have steam production only at a single pressure, non-reheat and thereby eliminating expensive evaporator sections with cylindrical, thick-walled drums and associated controls comprising valves and headers and reheat superheater sections with alloy steel finned tubes. Further reduction in complexity and cost comes from reduced length and footprint and requisite reinforced concrete foundation. Even further reduction in installed cost and complexity will be achieved via the simpler and less expensive steam turbine associated with the single-pressure steam produced in the one-pressure no reheat HRSG.